1. Field of the Invention
The present invention relates to a voltage comparator circuit which is used when an analog signal is converted into a digital signal in a semiconductor integrated circuit and, more particularly, to a voltage comparator circuit having hysteresis characteristics of a narrow range of voltage and using a high gain differential amplifier.
2. Description of the Related Art
FIG. 1 illustrates the arrangement of a conventional voltage comparator circuit. This circuit comprises a differential pair of NPN transistors Q11 and Q12 to which a reference voltage is applied, whose emitters are connected to each other, constant current source I10 connected between a connecting point of the emitters of transistors Q11 and Q12 and ground GND first resistor R11 connected between the collector of transistor Q11 and power source Vcc, PNP transistor Q13 whose emitter and collector are connected between power source Vcc and the collector of transistor Q12 and whose base and collector are connected to each other, PNP transistor Q14 which is connected to transistor Q13 to form a current mirror circuit and whose collector is connected to the base of transistor Q12, and second resistor R12 connected to the base of transistor Q12.
Reference voltage V.sub.B (threshold voltage) is applied to one end of second resistor R12, and comparison voltage Vi is applied to the base of transistor Q11. Comparison voltage Vo is output from the collector of transistor Q11.
The conventional voltage comparator circuit is caused to have hysteresis characteristics by applying a positive feedback to transistor Q12 from transistors Q13 and Q14. However, the hysteresis characteristics cannot be obtained unless the gain of the feedback is one or more. The gain is given as follows. EQU q .multidot.I10 .multidot.R12/4 .multidot.k .multidot.T (1)
The following is therefore the condition that the gain is one or more. EQU R12 &gt;4 .multidot.k T/q .multidot.I10 (2)
where k is the Boltzmann's constant, q is an amount of charge of electrons, and T is an absolute temperature.
It turns out from the above conditions that the hysteresis characteristics depend on k .multidot.T/q and thus have temperature dependency.
Since the voltage applying linearity of transistors Q11 and Q12 is poor and, in other words, a dynamic range is narrow, it is difficult to correlate R12 (or R12 I10 with a range of hysteresis.
As described above, the conventional voltage comparator circuit has a drawback in which it is virtually impossible to cause the circuit to reliably have the hysteresis characteristics of a narrow range of voltage.